1. Field of the Invention
The present invention relates to ring-shaped electrodes usable in high density memory devices based on programmable resistance material, like phase change based memory materials, and to methods for manufacturing such devices.
2. Description of Related Art
Chalcogenide materials are widely used in read-write optical disks. These materials have at least two solid phases, generally amorphous and generally crystalline. Laser pulses are used in read-write optical disks to switch between phases and to read the optical properties of the material after the phase change.
Chalcogenide materials also can be caused to change phase by application of electrical current. This property has generated interest in using programmable resistance material to form nonvolatile memory circuits.
In phase change memory, data is stored by causing transitions in the phase change material between amorphous and crystalline states using current. Current heats the material and causes transitions between the states. The change from the amorphous to the crystalline state is generally a lower current operation. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation. It is desirable to minimize the magnitude of the reset current used to cause transition of phase change material from crystalline state to amorphous state. The magnitude of the reset current needed for reset can be reduced by reducing the size of the active phase change material element in the cell. One problem associated with phase change memory devices arises because the magnitude of the current required for reset operations depends on the volume of phase change material that must change phase. Thus, cells made using standard integrated circuit manufacturing processes have been limited by the minimum feature size of manufacturing equipment. Thus, techniques to provide sublithographic dimensions for the memory cells must be developed, which can lack uniformity or reliability needed for large scale, high density memory devices.
One approach to controlling the size of the active area in a phase change cell is to devise very small electrodes for delivering current to a body of phase change material. This small electrode structure induces phase change in the phase change material in a small area like the head of a mushroom, at the location of the contact. See, U.S. Pat. No. 6,429,064, issued Aug. 6, 2002, to Wicker, “Reduced Contact Areas of Sidewall Conductor;” U.S. Pat. No. 6,462,353, issued Oct. 8, 2002, to Gilgen, “Method for Fabricating a Small Area of Contact Between Electrodes;” U.S. Pat. No. 6,501,111, issued Dec. 31, 2002, to Lowrey, “Three-Dimensional (3D) Programmable Device;” U.S. Pat. No. 6,563,156, issued Jul. 1, 2003, to Harshfield, “Memory Elements and Methods for Making Same.”
One type of bottom electrode structure that provides a small surface area contact is a ring-shaped electrode. See, Lai, U.S. Pat. No. 6,881,603, entitled Phase Change Material Memory Device, issued 19 Apr. 2005; Lowrey et al., U.S. Pat. No. 6,943,365, entitled Electrically Programmable Memory Element with Reduced Area of Contact and Method for Making Same issued 13 Sep. 2005; Lung, U.S. Patent Application Publication No. US 2007/0215852, entitled Manufacturing Method for Pipe-shaped Electrode Phase Change Memory, published on 20 Sep. 2007; and Lung, U.S. Patent Application Publication No. US 2008/0203375 entitled Memory Cell with Memory Element Contacting Ring-Shaped Upper End of Bottom Electrode. One benefit of ring-shaped electrodes arises from the geometry of the ring-shaped surface. Specifically, variations in diameters of the ring-shaped surface, and in thicknesses of the sidewalls on the pipe-shaped members at the ring-shaped surface have relatively little effect on the cell operational characteristics, allowing for more uniform distributions of cell characteristics across a large array than is available with other structures.
One problem with ring-shaped electrodes arises from the increased resistance due to the small cross-sectional area of the electrode orthogonal to the current flow direction. The increase in resistance requires a higher voltage across the memory cell to achieve a given current and increases the power consumption of the device. Also, in processes for forming a ring-shape electrode, thin-film sidewall structures are formed. It is difficult to manufacture such sidewall structures that have uniform thicknesses along the length of the electrode, particularly as the thicknesses of the sidewall structures used for the ring-shaped electrode fall below about 30 nanometers. Thus, thin spots can occur in the thin-film sidewalls which may result in unevenness in current flow around the ring, and even discontinuities in the structure that can affect manufacturing yield.
Accordingly, an opportunity arises to devise methods and structures that form memory cells with structures that have small active regions of programmable resistance material using reliable and repeatable manufacturing techniques.